Effects of temperature-dependent material properties and radioactive heat production on simple basin subsidence models

The effects of applying laboratory-derived material parameters to simple thermal basin subsidence models are examined. The temperature-dependent properties of thermal conductivity, specific heat and the coefficient of thermal expansion are considered, as well as conductivity contrasts between the crust and mantle and lithosphere-scale radiogenic heat production. The effects of conductivity and radioactivity are complementary, and can be predicted from the way in which they influence the initial and final steady-state geothermal gradient. A convex-up initial gradient, such as is generated by the conductivity and radioactivity functions and is also assumed to be the case based on independent evidence, will lead to more initial subsidence than in a simple constant-parameter model. The final subsidence will also be greater in the modified model, due to the fact that the stretching event will result in a lithospheric column which is intrinsically cooler than in the pre-deformation case. Because the coefficient of thermal expansion rises with increasing temperature, including its temperature dependence will result in a model with substantially less initial subsidence than one with a constant value. When these parameters are combined into a single model, the initial subsidence is approximately 15–20% less than in the constant-parameter model if radioactivity is not included, and 10% less if it is, while the final subsidence is about 5% greater without radioactivity and 7–9% greater with radioactivity included. Depending on the magnitude of extension, these effects can translate into differences of tens to hundreds of meters when compared to the constant-parameter model.     

La stratigraphie du Pleistocène supérieur et de l'Holocène dans le Golfe de Gascogne: Essai de synthèse des critères actuellement utilisables

Using analysis of marines cores from the Bay of Biscay as a basis, a system of different episodes of the Upper Pleistocene and the Holocene in the area of the Bay is presented. These episodes are characterized by several sedimentological and micropaleontological observations and particularly by the analysis of the coiling ratio of a few species of planktonic foraminifera. From the cores it is thus possible to determine the Holocene, the Würmian with its four glacial and three interglacial stages, and, at the base of the last glacial stage, the Riss-Würm interglacial or its upper level.     

Assessing uncertainty due to elevation error in a landslide susceptibility model

A landslide susceptibility model, employing a digital elevation model (DEM) and geological data, was used in a GIS to predict slope stability in a region of the H J Andrews Long-Term Research Forest, located in the Western Cascade Range in Oregon, USA. To evaluate the contribution of error in elevation to the uncertainty of the final output of the model, several different, but equally probable, versions of the input DEM were created through the addition of random, spatially autocorrelated noise (error) files. The realized DEMs were then processed to produce a family of slope stability maps from which the uncertainty effects of elevation error upon landslide susceptibility could be assessed. The ability to assess this uncertainty has the potential to help us better understand the inherent strengths and weaknesses of applying digital data and spatial information systems to this application, and to facilitate improved natural resource management decisions in relation to timber harvesting and slope stability problems.     

From non‐tidal shelf to tide‐dominated strait: The Miocene Bonifacio Basin,Southern Corsica

This study documents a change from a non‐tidal to tide‐dominated shelf system that occurred between Corsica and Sardinia (the Bonifacio Basin, Western Mediterranean) during the early to middle Miocene. The non‐tidal deposits formed on a low‐energy siliciclastic shelf surrounded by progradational coralline algal ramps at full highstand. The tidal deposits consist of an up to 200 m thick succession of siliciclastic to coralline‐rich cross‐beds formed by large sub‐tidal dunes. Based on outcrop and sub‐surface data, it is possible to conclude that the tidal currents were amplified as a consequence of the rapid subsidence of the basin centre due to tectonic activity. It is suggested that this tectonic event initiated the strait between Corsica and Sardinia. The strait was deep enough to allow the tidal flux to be significantly increased, generating a localized strong tidal current at the junction between the Western Mediterranean and the East Corsica Basin.     

Late Cretaceous to Miocene sea-level estimates from the New Jersey and Delaware coastal plain coreholes: an error analysis

Sea level has been estimated for the last 108 million years through backstripping of corehole data from the New Jersey and Delaware Coastal Plains. Inherent errors due to this method of calculating sea level are discussed, including uncertainties in ages, depth of deposition and the model used for tectonic subsidence. Problems arising from the two-dimensional aspects of subsidence and response to sediment loads are also addressed. The rates and magnitudes of sea-level change are consistent with at least ephemeral ice sheets throughout the studied interval. Million-year sea-level cycles are, for the most part, consistent within the study area suggesting that they may be eustatic in origin. This conclusion is corroborated by correlation between sequence boundaries and unconformities in New Zealand. The resulting long-term curve suggests that sea level ranged from about 75–110 m in the Late Cretaceous, reached a maximum of about 150 m in the Early Eocene and fell to zero in the Miocene. The Late Cretaceous long-term (10⁷ years) magnitude is about 100–150 m less than sea level predicted from ocean volume. This discrepancy can be reconciled by assuming that dynamic topography in New Jersey was driven by North America overriding the subducted Farallon plate. However, geodynamic models of this effect do not resolve the problem in that they require Eocene sea level to be significantly higher in the New Jersey region than the global average.     

Late Cretaceous and Cenozoic sea-level estimates: backstripping analysis of borehole data, onshore New Jersey

Backstripping analysis of the Bass River and Ancora boreholes from the New Jersey coastal plain (Ocean Drilling Project Leg 174AX) provides new Late Cretaceous sea‐level estimates and corroborates previously published Cenozoic sea‐level estimates. Compaction histories of all coastal plain boreholes were updated using porosity–depth relationships estimated from New Jersey coastal plain electric logs. The new porosity estimates are considerably lower than those previously calculated at the offshore Cost B‐2 well. Amplitudes and durations of sea‐level variations are comparable in sequences that are represented at multiple boreholes, suggesting that the resultant curves are an approximation of regional sea level. Both the amplitudes and durations of third‐order (0.5–5 Myr) cycles tend to decrease from the Late Cretaceous to the late Miocene. Third‐order sea‐level amplitudes in excess of 60 m are not observed. Long‐term (10⁸–10⁷ years) sea level was approximately constant at 30–80 m in the Late Cretaceous, rose to a maximum early Eocene value of approximately 100–140 m, and then fell through the Eocene and Oligocene.     

Brunhes time scales and the interpretation of climatic change

Two new δ¹⁸O time scales have been developed for the Brunhes Epoch using equatorial Pacific core V28-238. The first is based on a constant accumulation rate of aluminum, an assumption which has been shown to be acceptable for the last 360,000 years of the record by comparison with ²³⁰Th ages determined via the continuous strip-sample technique. The aluminum scale yields an age of 138,000 years for termination II and 693,000 years for the Brunhes-Matuyama reversal. Spectral and cross spectral analysis of the δ¹⁸O records of V28-238 and a detailed composite Indian Ocean record, using the aluminum time scale as well as two earlier time scales, indicates that the Pleistocene climate has been forced by periodic fluctuations of the earth's obliquity and precession. Based on this result, the second new time scale (TWEAQ) has been derived by tuning the δ¹⁸O record of V28-238 to the record of the earth's obliquity. TWEAQ yields an age of 127,000 years for termination II and 728,000 years for the Brunhes-Matuyama reversal. Spectral analysis of the δ¹⁸O record dated by TWEAQ indicates that 30% of the variance of the ice volume record can be ascribed to linear forcing by the earth's orbital parameters, but the trend of the data is consistent with a stochastic model.     

Thermally subsiding basins and the insulating effect of sediment with application to the Cambro-Ordovician Great Basin sequence, western USA

Tectonic subsidence of thermally generated basins is sensitive to the insulating effect of sediment. Compacting sediment reduces thermal subsidence, increases apparent stretching factors and reduces uncertainty in estimates of the breakup age. The transient effect of sediment insulation on the shape of the subsidence curve is considered by comparing model results with an exponential fit from 16 to 144 Myr after breakup. Misfits are dependent on the model parameters used, the degree of stretching, the degree of sediment compaction and the bottom boundary condition used in modelling. The magnitude of the misfit ranges up to 90 m (uncorrected for eustatic loading). These effects may alter the interpretation of backstripping results. Application to a data set from the Cambro-Ordovician miogeocline of the Great Basin, western USA, increases apparent stretching factors and reduces uncertainty in the predicted earliest Cambrian breakup age. In this case the misfits to exponential subsidence are quite large (?300 m) so that correction for the insulating effect of sediment does not eliminate a probable eustatic signal consistent with the Sauk sequence. If a eustatic signal is assumed, correction for model error suggests that the thermal parameters used are an improvement over those previously adopted and that the base of the lithosphere thins as sediments are added at the surface.